The present invention relates to films made from blends of thermoplastic polymer materials which have improved properties. More particularly the present invention relates to films made from blends of a propylene polymer with a butene-rich butene-1-propylene copolymer which are heat shrinkable and which have good clarity and good processability.
Thermoplastic blends for films are used as packaging material, and in the area of shrink packaging, for objects that are packaged in thermoplastic shrink film. Shrink film is used in many applications, for example, for many types of packaging and wrapping articles such as toys, sporting goods, stationary, greeting cards, hardware, household products, office supplies and forms, phonograph records, industrial parts, computer floppy diskettes, and photo albums, etc. Heat is applied to the film and the film shrinks to conform to the shape of the article packaged therein.
Many thermoplastic films shrink to some extent if they are subjected to elevated temperatures. Use is made of this characteristic by subjecting objects packaged in such films for a short time to elevated temperatures, e.g. exposing them to a blast of heated air, or by immersing in boiling water so that the film shrinks, thereby tightly enclosing the objects packaged therein. Examples are films fabricated from polyolefins or irradiated polyolefins.
Usually for most shrink film applications, a film should exhibit a high shrink energy or contractile force when exposed to elevated temperatures. In addition, the film should not only be heat shrinkable but have good clarity and be easily processed. Clarity is important in the marketing aspect of the packaged goods so that the consumer can ascertain what he or she is purchasing. Additional advantages of shrink packaging made from blends should include: (1) adds luster, enhances product appearance; (2) imparts contemporary image to the product; (3) helps the product sell itself; (4) keeps out dust and moisture so that the product doesn't become shop worn; (5) discourages shoplifters; (6) speeds production; (7) cuts labor and material costs; (8) reduces labeling cost; (9) eases the training of operators; (10) simplifies internal handling; (11) wraps unusual shapes with a contour fit; (12) versatile--serves many packaging needs; and (13) excellent for bundling and multipackaging.
A shrink film should possess the following specific properties:
(1) the shrink force should be between 100 and 400 grams per inch at 100.degree. C. depending on the objects to be encased. PA1 (2) the percent shrinkage should be between 10 and 50% at 121.degree. C. depending on the objects to be encased. PA1 (3) the film should have high clarity or optics. PA1 (4) the modulus should be between 60,000 and 350,000 psi depending upon the objects to be encased. PA1 (5) machinability: the coefficient of friction should be less than 0.5. PA1 (6) tear strength: the tear strength should be as high as possible; typical is 3 to 15 grams per mil of film thickness and per inch of width. PA1 (7) elongation: the elongation should be between 50 and 150% depending on the objects to be encased. PA1 from about 10% by weight to about 60% by weight butene-1-propylene copolymer where the propylene comonomer content of the butene-1-propylene copolymer is from about 5 m% to about 40 m%; and PA1 from about 40% by weight to about 90% by weight propylene homopolymer or copolymer.
Films may be oriented or unoriented. Oriented films may be obtained by stretching processes in which tensions capable of stretching the film are applied to the film, the directions of which form an angle of about 90.degree. utilizing well known prior art techniques. These film stretching tensions may be applied sequentially, as in the case if the film, after forming, is subjected to stretching in a longitudinal directions and thereafter tension is applied in a transverse direction to stretch the film transversely, or simultaneously, whereby longitudinal and transverse tensions are applied to the film at the same time resulting in a simultaneous longitudinal and transverse stretching of the film. Such processes are well known in the art and includes for example the "double-bubble" method which comprises extrusion of material into a tubular stalk, cooling of the tubular stalk, reheating and inflating of the tube and simultaneously drawing the inflated tube in a longitudinal direction thereby imparting biaxial orientation to the film. Another common method for the biaxial orientation of the film sheet comprises passing the film sheet through a series of rotating draw rollers which impart longitudinal directions stretch to the film and subsequently transversely drawing the longitudinally stretched film, for example, by passing it through a tenter frame wherein the film is stretched in a transverse direction.
In use, the film may be sealed around the product, formed into a bag, subjected to heat and shrunk tightly around the product. A variety of equipment is available for shrink packaging from manual as well as automatic systems. These systems use single-wound or centerfolded film. A sealing unit and a shrink tunnel are needed for shrink packaging. In a typical semi-automatic shrink packaging operation only three essential units are required--(1) a work surface for dispensing the film and inserting the product to be packaged, (2) an L-sealer, and (3) a heat tunnel.
For L-sealers, the film is supplied folded on a roll in widths to suit the need. By using folded film, the folded edge forms one side of the eventual package. The product to be packaged is inserted between the folded film and then moved to the heat sealing unit. Here an L-bar containing a hot wire descends in both heat seals and cuts the front edge and trailing edge. The leading edge has already been sealed during the heat sealing of the preceeding package. The product is now in a loose film fitting bag closed on all four sides. It next travels on a conveyer through a shrink tunnel where the film is exposed to heat resulting in a heated film shrinking tightly around the product to provide an attractive, clear form fitting package.
FIG. 1 illustrates a semi-automatic shrink packaging equipment system which may be used to "shrink" package articles. In FIG. 1, film 10 is provided by a roll. The film 10 is centerfolded film in this case. The centerfolded film is separated and the product 12 inserted. The product 12 is then moved to the heat sealing unit 14 where an L-bar 16 containing a hot wire descends and both heat seals and cuts the front edge and trailing edge of the package 18. The package 18 travels on a conveyer 20 through a shrink tunnel 22 where the film is exposed to heat. The resulting shrunk package 24 leaves tunnel 22 by way of conveyer 20.
U.S. Pat. No. 3,900,534 discloses a biaxially oriented thermoplastic film structure formed from a blend comprising polypropylene and polybutene homopolymers where the polybutene is present in a small amount of more than 10% but less than 20% by weight.
U.S. Pat. No. 3,634,553 discloses a heat shrinkable oriented thermoplastic film which comprises a blend of polypropylene and an ethylene/butene-1 copolymer.
European patent application No. 0,145,014A discloses a blend of a random copolymer of propylene and an alpha olefin with 4 or more carbon atoms (i.e. perhaps butene-1), where the alpha olefin content in the copolymer is 8 to 30 mole% (m%).
Single layer shrink films based on blends of polybutylene with polypropylene are disclosed in Mobil Patents: U.S. Pat. No. 3,634,552 (1972), U.S. Pat. No. 3,634,553 (1972), U.S. Pat. No. 3,849,520 (1974) and U.S. Pat. No. 3,900,534 (1975) and blends of polybutylene with ethylene vinyl acetate (EVA) and C.sub.2 -C.sub..alpha. elastomer or polybutylene with low density polyethylene (LDPE) and C.sub.2 -C.sub..alpha. elastomer (where C.sub..alpha. is an .alpha.-olefin comonomer) are disclosed in U.S. Pat. No. 4,379,888 (1983). Multilayers may include three layers (propylene-ethylene plus butene-1-ethylene plus ethylene-propylene rubber)/tie layer/linear low density polyethylene (LLDPE) by Union Carbide, U.S. Pat. No. 4,196,240 (1980) for frozen poultry and U.S. Pat. No. 4,207,363 (1980) for primal meat cuts. Three layers of propylene-ethylene/EVA+butene-1-ethylene)/propylene-ethylene, U.S. Pat. No. 4,194,039 (1980) is known. Also, three layers (polypropylene+polybutylene)/EVA/irradiated EVA by Cryovac, U.S. Pat. No. 3,754,063 (1973), U.S. Pat. No. 3,832,274 (1974), and U.S. Pat. No. 3,891,008 (1975) for turkey bags are known.
Heretofore, polyvinyl chloride (PVC) has been used to produce good shrink films. PVC has been shown to be much better in certain applications than the use of polyolefins such as propylene polymers. This is because the use of polyolefins in shrink wrap results in a moderate to high shrink force which is undesirable in many applications. However, the use of polyolefins allows for the use of high speed automated packaging machinery with ease of control, lower cost, and less deposit from corrosion of equipment, which results in less equipment maintenance than when using PVC. PVC, however, may produce a better looking package because of the low shrink force and better optics. Also, the seal and shrink of PVC films may take place over a much broader temperature range and tear strength may be better.
It has been desired to produce a blend for producing a heat shrinkable thermoplastic film with the film advantages of PVC but which is of low cost, can be used on a high speed automated packaging machine and which does not corrode equipment. It is the butene-rich butene-1-propylene polymer blend film of the present invention which results in low shrink force which is adjustable by the blending ratio, low shrink temperature, low stiffness with better optics, and which does not corrode the equipment being used, is of lower cost and can be used on high speed automated packaging machines.